1. Field of the Invention
This invention relates to a transmission with an output gear for moving axially a threaded rod, which is engaged threadably within the output gear, more particularly to a transmission with a thrust bearing, which facilitates smooth axial movement of the threaded rod.
2. Description of the Related Art
The improvement of this invention is directed to a conventional transmission, which is used to adjust a slope of a foot-supporting member of a treadmill that serves as a leg exerciser. Referring to FIG. 1, the conventional transmission 10 is shown to include a gear box 11 in which a reduction gearing 12 is disposed. The reduction gearing 12 includes three unitary speed-reducing elements 13, 14, 15, and an output gear 16. Each of the speed-reducing elements 13, 14, 15 has a large gear 131, 141, 151, and a small gear 132, 142, 152. When a motor 90 is powered, a bevel gear 91 that is fixed on a motor shaft of the motor 90 rotates the large gear 131 so that the small gear 152 rotates the output gear 16. The bevel gear 91 has an end 911, which is biased by a spring 92 to press a washer 18 against a side surface of the large gear 141. As such, when power to drive the motor 90 is switched off, rotation of the elements 13, 14, 15 stops. The output gear 16 has an integral shaft 161, which is formed with a threaded axial hole 162. An externally threaded rod 17 is engaged within the threaded axial hole 162 in the shaft 161. Accordingly, when power to drive the motor 9 is switched on, rotation of the bevel gear 91 is converted into axial movement of the threaded rod 17. The aforementioned conventional transmission suffers from the following disadvantages:
(1) Because there is no bearing provided between the threaded rod 17 and the gear box 11, the threaded rod 17 cannot move smoothly in the gear box 11. Furthermore, friction between external threads on the rod 17 and internal threads in the shaft 161 reduces largely the output torque of the transmission 12. PA1 (2) It is more difficult to effect upward movement of the threaded rod 17 than downward movement of the same due to the weight of the load that is carried on the top end of the rod 17. Accordingly, the transmission 12 is equipped normally with a control circuit device (not shown), which is selected according to the type of the load, thereby increasing the manufacturing costs.